A direct positive silver halide photographic material can be obtained by using a previously light-fogged or chemical-fogged emulsion taking advantage of solarization. For example, addition of an electron acceptor to a silver halide photographic emulsion having been fogged by a reducing agent combined with a metallic compound affords a direct positive silver halide photographic emulsion as described, e.g., in British Patent 723,019 and U.S. Pat. No. 3,501,307.
Known electron acceptors to be added to a fogged emulsion include pyridinium or bipyridinium salt derivatives as described in British Patents 871,938, 873,937 and 875,887, U.S. Pat. Nos. 3,583,870 and 3,945,832, and JP-B-56-47544 (the term "JP-B" as used herein means an "examined Japanese patent publication"); 4-nitropyridine oxide as described in British Patent 905,237; nitro-substituted benzimidazole as described in British Patent 907,367; nitrophenyl-containing thiazoline-2-thion compounds as described in U.S. Pat. No. 3,367,779; cyanine dyes having an indole nucleus as described in U.S. Pat. Nos. 2,930,694, 3,314,796, 3,501,310, 3,501,311, 3,501,312, 3,505,070, and 3,687,675, and British Patent 970,601; dyes containing an imidazo[4,5b]quinoxaline nucleus as described in U.S. Pat. No. 3,431,111; cyanine dyes having a desensitizing substituent, such as a nitro group, on their two heterocyclic nuclei as described in British Patent 723,019; quaternarized merocyanine dyes as described in U.S. Pat. Nos. 3,539,349 and 3,574,629; pyrylium, thiapyrylium and selenapyrylium dyes as described in U.S. Pat. No. 3,579,345; cyanine dyes containing a 4-pyrazole nucleus as described in U.S. Pat. No. 3,615,608; polymethine dyes containing an imidazole nucleus as described in U.S. Pat. No. 3,615,639; and trinuclear tetramethine cyanine dyes as described in JP-B-1-13091.
IN the field of a photomechanical process where formation of a sharp dot image or line image is required, an image formation system providing extremely high contrast photographic characteristics (gamma of at least 10) is demanded. However, the above-mentioned direct positive silver halide photographic material having added thereto an electron acceptor, though exhibiting high sensitivity, has only achieved a low contrast, failing to form a satisfactory dot image or line image.
In order to eliminate the above disadvantage, various image formation systems have been proposed to date. For example, U.S. Pat. No. 3,615,517 proposes a system comprising developing a pre-fogged silver halide photographic material having added thereto an organic electron acceptor whose oxidation potential and reduction potential sum up to a positive value and a halogen transfer compound whose oxidation potential is less than 0.85 V and whose reduction potential is less than -1.0 V with a developer containing a single developing agent of hydroquinone. U.S. Pat. No. 3,615,519 discloses a pre-fogged silver halide photographic material having added thereto an alkylene oxide polymer. U.S. Pat. No. 3,632,340 suggests a system in which a pre-fogged silver halide photographic material having an internal center accelerating the precipitation of photodecomposed silver is developed with a developer containing a single developing agent of hydroquinone. Further, JP-A-54-4118 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-54-9620, JP-B-58-5420, JP-B-1-13090, and U.S. Pat. No. 4,273,862 teach a system in which a pre-fogged silver halide photographic material containing an anionic cyanine dye whose half wave oxidation potential is more positive than +0.4 V in polarography is developed with a lith developer.
However, although the contrast of these image formation systems is higher as compared with other methods, the contrast level is still insufficient for use in a photomechanical process.